This invention relates to a magnetic core (which will hereinunder be often referred to as xe2x80x9ccorexe2x80x9d simply) which is used in an inductance element such as a choke coil and a transformer for use in a switching power supply or the like and, in particular, to a magnetic core comprising a permanent magnet for magnetically biasing.
In a choke coke and a transformer used in, for example, a switching power supply or the like, a voltage is usually applied thereto with an AC component superposed to a DC component. Therefore, a magnetic core used in those choke coil and transformer is required to have a magnetic characteristic of a good magnetic permeability so that the core is not magnetically saturated by the superposition of the DC component. This magnetic characteristic will be referred to as xe2x80x9cDC superposition characteristicxe2x80x9d or simply xe2x80x9csuperposition characteristicxe2x80x9d in the art.
As magnetic cores in application fields within high frequency bands, there have been used a ferrite magnetic core and a dust magnetic core. These magnetic cores have individual features due to physical properties of their materials. That is, the ferrite magnetic core has a high intrinsic magnetic permeability and a low saturated magnetic flux density while the dust magnetic core has a low intrinsic magnetic permeability and a high saturated magnetic flux density. Accordingly, the dust magnetic core is often used as one having a toroidal shape. On the other hand, the ferrite magnetic core has an E-shape core part having a central leg formed with a magnetic gap so as to prevent magnetic saturation from being caused by the superposition of the DC component.
Recently, since electronic parts are required to be small-sized as electronic devices are more compact-sized, the magnetic core with the magnetic gap is small-sized too. So, there is a strong demand for magnetic cores having an increased magnetic permeability against superposition of DC component.
Generally, it is necessary for the demand to select a magnetic core having a high saturation magnetization, that is, to select a magnetic core that is not magnetically saturated by a high magnetic field applied. However, the saturation magnetization is inevitably determined by materials and cannot be made as high as desired.
As a solution, it has been conventionally proposed to dispose a permanent magnet in a magnetic gap formed in a magnetic path of a magnetic core, that is, to magnetically bias the magnetic core, to thereby cancel a DC magnetic flux caused by the superposition of DC component.
The magnetic bias by use of the permanent magnet is a good solution to improve the DC superposition characteristic. However, this method have hardly been brought into a practical use for reasons as follows. More specifically, use of a sintered metallic magnet resulted in considerable increase of a core loss of the magnetic core. In addition, use of a ferrite magnet led in unstable superposition characteristic.
Means to resolve the problems is disclosed, for example, in Japanese Unexamined Patent Publication No. S50-133453 or JP 50-133453 A. This Publication uses, as a magnetically biasing magnet, a bond magnet comprising rare-earth magnetic powder with a high magnetic coercive force and binder which are mixed together with each other and compacted into a shape. Thereby, the DC superposition characteristic and temperature elevation of the core are improved.
Recently, a power supply has been more and more strongly required to improve its power transformation efficiency. Accordingly, this requirement has been became to a high level that it is difficult to determine good and bad of magnetic cores for choke coils and transformers by core temperatures measured. It is therefore inevitable to determine it from core loss data measured by use of a core-loss measuring device. According to the study by the present inventors, it was confirmed that the core loss has a degraded value in cores having the resistance value disclosed in JP 50-133453 A.
It is therefore a first object of this invention to provide, in a magnetic core which has at least one magnetic gap formed in a magnetic path and which comprises a magnetically biasing magnet disposed in the vicinity of the magnetic gap for providing a magnetic bias from opposite ends of the magnetic gap to the core, easily and cheaply the magnetic core having an excellent DC superposition characteristic and an excellent core-loss characteristic in consideration of the above description.
In addition, there have recently been demands for coil parts of a surface-mounted type. Those coil parts are subjected to reflow soldering process so as to be surface-mounted on a circuit board. It is desired that a magnetic core of the coil part be not degraded in its magnetic properties under conditions of the reflow soldering process. Further, an oxidation-resistant rare-earth magnet is indispensable.
It is a second object of this invention to provide, in a magnetic core which has at least one magnetic gap formed in a magnetic path and which comprises a magnetically biasing magnet disposed in the vicinity of the magnetic gap for providing a magnetic bias from opposite ends of the magnetic gap to the core, easily and cheaply the magnetic core which has an excellent DC superposition characteristic, an excellent core-loss characteristic, and oxidation resistance without affecting the characteristics under conditions of the reflow soldering process in consideration of the above description.
Furthermore, it is desired that not only magnetic powder has an improved oxidation resistance but also a rare-earth magnet has a high specific resistance.
It is a third object of this invention to provide, in a magnetic core which has at least one magnetic gap formed in a magnetic path and which comprises a magnetically biasing magnet disposed in the vicinity of the magnetic gap for providing a magnetic bias from opposite ends of the magnetic gap to the core, easily and cheaply the magnetic core which has an excellent DC superposition characteristic, an excellent core-loss characteristic, oxidation resistance, and a high specific resistance in consideration of the above description.
According to a first aspect of this invention, in order to achieve the above-mentioned first object in a magnetic core which has at least one magnetic gap formed in a magnetic path and which comprises a magnetically biasing magnet disposed in the vicinity of the magnetic gap for providing a magnetic bias from opposite ends of the magnetic gap to the core, there is provided the magnetic core comprising the magnetically biasing magnet, wherein the magnetically biasing magnet comprises a bond magnet comprising rare-earth magnetic powder and a binder resin, the rare-earth magnetic powder has an intrinsic coercive force of 5 kOe or more, a Curie temperature of 300xc2x0 C. or more, and an average particle size of 2.0-50 xcexcm, and the rare-earth magnetic powder consists of an aggregation of magnetic particles surfaced with a coating of a metallic layer containing an oxidation-resistant metal.
Preferably, the oxidation-resistant metal may be, for example, at least one metal or alloy thereof selected from a group of zinc, aluminum, bismuth, gallium, indium, magnesium, lead, antimony, tin.
Preferably, the bond magnet may comprise the binder resin content thereof which is 20% or more on the base of a volumetric percentage and the bond magnet may have a specific resistance of 1 xcexa9xc2x7cm or more. The binder resin may be polyamideimide resin.
In addition, the magnetic powder preferably may comprise the oxidation-resistant metal content thereof which is 0.1-10% on the base of a volumetric percentage.
Furthermore, it is possible to obtain an inductance part by winding at least one winding by one or more turns on the above-mentioned magnetic core comprising the magnetically biasing magnet.
In addition, the inductance part includes a coil, a choke coil, a transformer, and other parts each of which generally essentially comprises a core and winding or windings.
According to a second aspect of this invention, in order to achieve the above-mentioned second object in a magnetic core which has at least one magnetic gap formed in a magnetic path and which comprises a magnetically biasing magnet disposed in the vicinity of the magnetic gap for providing a magnetic bias from opposite ends of the magnetic gap to the core, there is provided the magnetic core comprising the magnetically biasing magnet, wherein the magnetically biasing magnet comprises a bond magnet which comprises rare-earth magnetic powder and a binder resin, the rare-earth magnetic powder has an intrinsic coercive force of 10 kOe or more, a Curie temperature of 500xc2x0 C. or more, and an average particle size of 2.5-50 xcexcm, and the rare-earth magnetic power consists of an aggregation of magnetic particles surfaced with a coating of a metallic layer containing an oxidation-resistant metal.
Preferably, the oxidation-resistant metal may be, for example, at least one metal or alloy thereof selected from a group of zinc, aluminum, bismuth, gallium, indium, magnesium, lead, antimony, tin.
Preferably, the bond magnet may comprise the binder resin content thereof which is 30% or more on the base of a volumetric percentage and the bond magnet may have a specific resistance of 1 xcexa9xc2x7cm or more. The binder resin may be polyamideimide resin.
In addition, the magnetic powder preferably may comprise the oxidation-resistant metal content thereof which is 0.1-10% on the base of a volumetric percentage.
Furthermore, it is possible to obtain an inductance part by winding at least one winding by one or more turns on the above-mentioned magnetic core comprising the magnetically biasing magnet.
In addition, the inductance part includes a coil, a choke coil, a transformer, and other parts each of which generally essentially comprises a core and winding or windings.
According to a third aspect of this invention, in order to achieve the above-mentioned third object in a magnetic core which has at least one magnetic gap formed in a magnetic path and which comprises a magnetically biasing magnet disposed in the vicinity of the magnetic gap for providing a magnetic bias from opposite ends of the magnetic gap to the core, there is provided the magnetic core comprising the magnetically biasing magnet, wherein the magnetically biasing magnet comprises a bond magnet which comprises rare-earth magnetic powder and a binder resin, the rare-earth magnetic powder has an intrinsic coercive force of 10 kOe or more, a Curie temperature of 500xc2x0 C. or more, and an average particle size of 2.5-50 xcexcm, the bond magnet comprises the binder resin content thereof which is 30% or more on the base of a volumetric percentage, the bond magnet has a specific resistance of 1 xcexa9xc2x7cm or more, and the rare-earth magnetic power consists of an aggregation of magnetic particles surfaced with a coating of a metallic layer containing an oxidation-resistant metal, the metallic layer is surfaced with a coating of a glass layer consisting of low-melting glass having a softening point which is lower than a melting point of the oxidation-resistant metal.
Preferably, the oxidation-resistant metal may be, for example, at least one metal or alloy thereof selected from a group of zinc, aluminum, bismuth, gallium, indium, magnesium, lead, antimony, tin.
Preferably, the magnetic powder may comprise the oxidation-resistant metal and the said low-melting glass total content thereof which is 0.1-10% on the base of a volumetric percentage. The said binder resin may be polyamideimide resin.
Furthermore, it is possible to obtain an inductance part by winding at least one winding by one or more turns on the above-mentioned magnetic core comprising the magnetically biasing magnet.
In addition, the inductance part includes a coil, a choke coil, a transformer, and other parts each of which generally essentially comprises a core and winding or windings.
The present co-inventors first studied a permanent magnet to be inserted to achieve the above-mentioned first object of this invention. The co-inventors resultantly obtained a knowledge that a use of a permanent magnet having a specific resistance of 1 xcexa9xc2x7cm or more and an intrinsic coercive force iHc of 5 kOe or more can provide a magnetic core which has an excellent DC superposition characteristic and a non-degraded core-loss characteristic. This means that the property of the magnet necessary for obtaining an excellent DC superposition characteristic is the intrinsic coercive force rather than the energy product. Thus, this invention is based on the findings that it is possible to provide a sufficient high DC superposition characteristic if a permanent magnet has a high intrinsic coercive force although the permanent magnet having a high specific resistance is used.
The permanent magnet having a high specific resistance and a high intrinsic coercive force can be generally realized by a rare-earth bond magnet which is formed of rare-earth magnetic powder and a binder mixed together, then compacted. However, the magnetic powder used may be any kind of magnetic powder having a high coercive force. The rare-earth magnetic powder includes SmCo series, NdFeB series, SmFeN series, and other.
A magnetic core for a choke coil or a transformer can be effectively made of any kind of materials which have a soft magnetism. Generally speaking, the materials include ferrite of MnZn series or NiZn series, dust magnetic core, silicon steel plate, amorphous or others. Further, the magnetic core is not limited to a special shape but this invention can be applicable to a magnetic core having a different shape such as toroidal core, E-E core, E-l core or others. Each of these magnetic cores has at least one magnetic gap in its magnetic path in which gap the permanent magnet is disposed.
Although the gap is not restricted in a length thereof, the DC superposition characteristic is degraded when the gap length is excessively small. When the gap length is, on the other hand, excessively large, the permeability is lowered. Accordingly, the gap length is determined automatically. Although it is easily possible to obtain a bias effect if a magnetically biasing permanent magnet has a larger thickness, the magnetically biasing permanent magnet preferably may have a smaller thickness for miniaturization of a magnetic core. However, it is difficult to obtain a sufficient magnetic bias if the thickness of the magnetically biasing permanent magnet is smaller than 50 xcexcm. Accordingly, a length of 50 xcexcm or more is required for the magnetic gap in which the magnetically biasing permanent magnet is disposed and a length of 10000 xcexcm or less may be preferable in respect of restraint of a size in the core.
As regards a requirement character for a permanent magnet inserted in a magnetic gap, an intrinsic coercive force of 5 kOe or more is required. This is because a coercive force disappears caused by a DC magnetic field applied to a magnetic core if the intrinsic coercive force is 5 kOe or less. In addition, although a specific resistance preferably may be high, degradation of a core-loss is not caused by the specific resistance if the specific resistance has 1 xcexa9xc2x7cm or more. In addition, the average particle size of the magnetic powder is desired 50 xcexcm or less at the maximum because the use of the magnetic powder having the average particle size larger than 50 xcexcm results in degradation of the core-loss characteristic. While the minimum value of the average particle size is required 2.0 xcexcm or more because the powder having the average particle size less than 2.0 xcexcm is significant in magnetization reduction due to oxidation of particle caused by grinding.
Herein, in order to improve oxidation resistance in magnetic powder, the magnetic powder desirably may consist of an aggregation of magnetic particles surfaced with a coating of an oxidation-resistant metal which is at least one metal or alloy thereof selected from a group of zinc, aluminum, bismuth, gallium, indium, magnesium, lead, antimony, tin. It is possible to obtain a magnetic core which copes with both oxidation resistance and a high DC superposition characteristic if the amount of the oxidation-resistant metal lies between 0.1-10% on the base of volumetric percentage.
In addition, the present co-inventors studied a permanent magnet to be inserted to achieve the above-mentioned second object of this invention. The co-inventors resultantly obtained a knowledge that a use of a permanent magnet having a specific resistance of 1 xcexa9xc2x7cm or more and an intrinsic coercive force iHc of 10 kOe or more can provide a magnetic core which has an excellent DC superposition characteristic and a non-degraded core-loss characteristic. This means that the property of the magnet necessary for obtaining an excellent DC superposition characteristic is the intrinsic coercive force rather than the energy product. Thus, this invention is based on the findings that it is possible to provide a sufficient high DC superposition characteristic if a permanent magnet has a high intrinsic coercive force although the permanent magnet having a high specific resistance is used.
The permanent magnet having a high specific resistance and a high intrinsic coercive force can be generally realized by a rare-earth bond magnet which is formed of rare-earth magnetic powder and a binder mixed together, then compacted. However, the magnetic powder used may be any kind of magnetic powder having a high coercive force. Although the rare-earth magnetic powder includes SmCo series, NdFeB series, SmFeN series, and other, in the present circumstances, it is restricted to Sm2Co17 series magnet because a magnet having a Curie temperature Tc of 500xc2x0 C. and a coercive force of 10 kOe or more is required in consideration of conditions of the reflow soldering process and the oxidation resistance.
A magnetic core for a choke coil or a transformer can be effectively made of any kind of materials which have a soft magnetism. Generally speaking, the materials include ferrite of MnZn series or NiZn series, dust magnetic core, silicon steel plate, amorphous or others. Further, the magnetic core is not limited to a special shape but this invention can be applicable to a magnetic core having a different shape such as toroidal core, E-E core, E-l core or others. Each of these magnetic cores has at least one magnetic gap in its magnetic path in which gap the permanent magnet is disposed.
Although the gap is not restricted in a length thereof, the DC superposition characteristic is degraded when the gap length is excessively small. When the gap length is, on the other hand, excessively large, the permeability is lowered. Accordingly, the gap length is determined automatically. Although it is easily possible to obtain a bias effect if a magnetically biasing permanent magnet has a larger thickness, the magnetically biasing permanent magnet preferably may have a smaller thickness for miniaturization of a magnetic core. However, it is difficult to obtain a sufficient magnetic bias if the thickness of the magnetically biasing permanent magnet is smaller than 50 xcexcm. Accordingly, a length of 50 xcexcm or more is required for the magnetic gap in which the magnetically biasing permanent magnet is disposed and a length of 10000 xcexcm or less may be preferable in respect of restraint of a size in the core.
As regards a requirement character for a permanent magnet inserted in a magnetic gap, an intrinsic coercive force of 10 kOe or more is required. This is because a coercive force disappears caused by a DC magnetic field applied to a magnetic core if the intrinsic coercive force is 10 kOe or less. In addition, although a specific resistance preferably may be high, degradation of a core-loss is not caused by the specific resistance if the specific resistance has 1 xcexa9xc2x7cm or more. In addition, the average particle size of the magnetic powder is desired 50 xcexcm or less at the maximum because the use of the magnetic powder having the average particle size larger than 50 xcexcm results in degradation of the core-loss characteristic. While the minimum value of the average particle size is required 2.5 xcexcm or more because the powder having the average particle size less than 2.5 xcexcm is significant in magnetization reduction due to oxidation of particle caused by grinding.
Herein, in order to improve oxidation resistance in magnetic powder, the magnetic powder desirably may consist of an aggregation of magnetic particles surfaced with a coating of an oxidation-resistant metal which is at least one metal or alloy thereof selected from a group of zinc, aluminum, bismuth, gallium, indium, magnesium, lead, antimony, tin. It is possible to obtain a magnetic core which copes with both oxidation resistance and a high DC superposition characteristic if the amount of the oxidation-resistant metal lies between 0.1-10% on the base of volumetric percent.
Furthermore, the present co-inventors studied a permanent magnet to be inserted to achieve the above-mentioned third object of this invention. The co-inventors resultantly obtained a knowledge that a use of a permanent magnet having a specific resistance of 1 xcexa9xc2x7cm or more and an intrinsic coercive force iHc of 10 kOe or more can provide a magnetic core which has an excellent DC superposition characteristic and a non-degraded core-loss characteristic. This means that the property of the magnet necessary for obtaining an excellent DC superposition characteristic is the intrinsic coercive force rather than the energy product. Thus, this invention is based on the findings that it is possible to provide a sufficient high DC superposition characteristic if a permanent magnet has a high intrinsic coercive force although the permanent magnet having a high specific resistance is used.
The permanent magnet having a high specific resistance and a high intrinsic coercive force can be generally realized by a rare-earth bond magnet which is formed of rare-earth magnetic powder and a binder mixed together, then compacted. However, the magnetic powder used may be any kind of magnetic powder having a high coercive force.
Although the rare-earth magnetic powder includes SmCo series, NdFeB series, SmFeN series, and other, in the present circumstances, it is restricted to Sm2Co17 series magnet because a magnet having a Curie temperature Tc of 500xc2x0 C. and a coercive force of 10 kOe or more is required in consideration of conditions of the reflow soldering process and the oxidation resistance.
A magnetic core for a choke coil or a transformer can be effectively made of any kind of materials which have a soft magnetism. Generally speaking, the materials include ferrite of MnZn series or NiZn series, dust magnetic core, silicon steel plate, amorphous or others. Further, the magnetic core is not limited to a special shape but this invention can be applicable to a magnetic core having a different shape such as toroidal core, E-E core, E-l core or others. Each of these magnetic cores has at least one magnetic gap in its magnetic path in which gap the permanent magnet is disposed.
Although the gap is not restricted in a length thereof, the DC superposition characteristic is degraded when the gap length is excessively small. When the gap length is, on the other hand, excessively large, the permeability is lowered. Accordingly, the gap length is determined automatically.
As regards a requirement character for a permanent magnet inserted in a magnetic gap, an intrinsic coercive force of 10 kOe or more is required. This is because a coercive force disappears caused by a DC magnetic field applied to a magnetic core if the intrinsic coercive force is 10 kOe or less. In addition, although a specific resistance preferably may be high, degradation of a core-loss is not caused by the specific resistance if the specific resistance has 1 xcexa9xc2x7cm or more. In addition, the average particle size of the magnetic powder is desired 50 xcexcm or less at the maximum because the use of the magnetic powder having the average particle size larger than 50 xcexcm results in degradation of the core-loss characteristic. While the minimum value of the average particle size is required 2.5 xcexcm or more because the powder having the average particle size less than 2.5 xcexcm is significant in magnetization reduction due to oxidation of particle caused by grinding.
Herein, in order to improve oxidation resistance in magnetic powder, the magnetic powder desirably may consist of an aggregation of magnetic particles surfaced with a coating of an oxidation-resistant metal which is at least one metal or alloy thereof selected from a group of zinc, aluminum, bismuth, gallium, indium, magnesium, lead, antimony, tin. However, it seems obvious to those skilled in the art that it results in bringing on degradation of a specific resistance when the surface of each magnetic particle in the magnetic powder is coated with the oxidation-resistant metal. The specific resistance preferably may be high from the point of view of efficiency in a power supply and frequency characteristics in magnetic permeability xcexc. In order to improve the specific resistance, coating of the oxidation-resistant metal is surfaced with a coating of a low-melting glass having a softening point which is lower than a melting point of the oxidation-resistant metal in question. Thus, it is possible to obtain a magnetic core which copes with both a high specific resistance and oxidation resistance. The oxidation-resistant and the low-melting glass total content of the magnetic powder may be desired 0.1% or more on the base of volumetric percentage because oxidation resistance is substantially equivalent to additive-free if the oxidation-resistant and the low-melting glass total content of the magnetic powder is less than 0.1% on the base of volumetric percentage. In addition, the total content may be desired 10% or less on the base of volumetric percentage because the magnetic powder has a low packing factor and a decreased magnetic flux if the total content is more than 10%. Accordingly, it is possible to obtain a magnetic core which copes with both oxidation resistance and a high specific resistance when the oxidation-resistant and the low-melting glass total content of the magnetic powder lies between 0.1-10% on the base of volumetric percentage.